Here are some recent ones that come to mind:
Arian Foster--he was a 3 stat out of HS and I think he has the 2nd most rushing yards in UT history and is one of the best RBs and players in the NFL.
Denarius Moore--he was also a 3 star out of HS and thought to be the 2nd half of a package deal with Lennon Creer. He ended up being a heck of a receiver for UT and had a huge senior season. He's also doing well in the NFL as the number 1 receiver for the Raiders.
Nick Reveiz--he wasn't the most physically gifted guy to play for UT and is not playing in the NFL like the other two guys. But he got the most out of the talent he did have and ended up being a good LB for UT and a big-time leader for our defense and team as a whole. I wish we had a guy like him to lead our team every year.
I'm not saying they are the top over-achievers in the history of UT. They are just some of the recent ones who came to mind and were all very good players for UT.